(1) Field of the Invention
The present invention relates to acoustic transducers, and more particularly, to an intermediary impedance for an acoustic transducer and an acoustic signal medium.
(2) Description of the Prior Art
Acoustic transducers may be used to transmit or receive an acoustic signal in a medium. The specific acoustic impedance of a medium is the ratio of the acoustic pressure to the acoustic particle velocity of sound propagating through the medium. Typically, transducers, such as piezoelectric transducers incorporating PZT (lead zirconate titanate), have a specific acoustic impedance much larger than the specific acoustic impedance of the medium through which an acoustic signal is transmitted or received by the transducer. This impedance mismatch may result in a significant amount of reflected energy, and resultant signal loss, at the boundary between the transducer and medium.
A layer of material that serves as an acoustic impedance matching layer may be inserted at the boundary between the transducer and the surrounding acoustic medium in order to improve signal propagation from the transducer to the medium or vice versa. Specifically, an acoustic impedance matching layer having an impedance intermediate of the transducer and the medium may be inserted at the boundary to reduce the amount of reflected acoustic energy. Prior art matching layers include screens, perforated plates, and various layers of polymers with inclusions (see e.g., U.S. Pat. No. 3,674,945 to Hands).
These solutions typically provide one or more so-called static layers (i.e., layers that do not move). Problems have been identified with these solutions. For example, static layers may support shear waves which travel transversely to the direction of acoustic signal propagation and have been associated with compromising desired aspects of the acoustic signal. Further, static layers are often times incapable of being removed, supplemented, or otherwise adjusted to provide a user with the capacity to implement a desired level of impedance matching. Also, the limits of known materials for static layers may prevent users from implementing a desired matching impedance.
What is needed then is a solution for impedance mismatch between a transducer and a surrounding acoustic medium which provides a user with the capacity to implement a desired impedance match, to allow a user to adjust such impedance, and to prevent the introduction of potentially compromising shear waves.